Back up power systems are available which are based on several different topologies. While many of these are sometimes referred to as uninterruptible power systems, strictly speaking they are not. In these so called UPS systems, power from a first source, most typically an a.c. power line, powers the load either directly or through a conditioning or regulating device such as a ferroresonant transformer. Upon failure of the first source, a second source, typically an inverter powered by batteries, is actively switched into the circuit to supply the load. Sometimes the first and second sources share a single ferroresonant transformer so that the load is always powered by the ferroresonant transformer, which in turn is powered by either the first source or, upon failure of the first source, the second power source which is switched in to power the transformer. Two factors conspire to prevent such designs from providing truly uninterruptible power. First, it is necessary to monitor the first power source continuously, and quickly determine that it has failed. Given the inherent time variant nature of the sinusoidal wave form typical of a.c. power sources, this is very difficult to achieve. Further, once it has been determined that the first power source has failed, it is necessary to switch over to the second power source and to positively disconnect the first source to assure that power is not fed back to the first source from the second. Typically, this switch over and disconnect process is accomplished using an electromechanical relay or contactor, which is inherently a slow device. Allowing for the time required to first detect failure of the first power source and then accomplish the change over to the second source, the output power may be interrupted for several tens of milliseconds. This is acceptable for some loads, but not others.
For sensitive loads, a full time inverter, i.e., a true UPS, is best. In this known topology, a single inverter converts d.c. power to a.c. power and supplies it to the load on a continuous basis. Generally, power is normally supplied from a first a.c. power supply, normally a power line, through a rectifier to provide a first d.c. source which normally supplies the d.c. power required by the inverter. This same rectifier keeps charged batteries which provide a second d.c. source of power. The second d.c. source is connected to the inverter in parallel with the first d.c. source. Upon failure of the first d.c. source, the inverter continues to be supplied with d.c. power from the batteries. Upon restoration of the first d.c. source, the rectifier once again powers the inverter and, at the same time, recharges the batteries. Because of the parallel connection of the first source and the battery across the input to the inverter, the inverter always supplies a.c. power to the load with no disturbance or interruption of the a.c. output when the first source fails or is restored.
The advantages of a true UPS include a continuously uninterrupted power output to the load, as well as an output whose frequency and wave form are independent of the input. This allows the UPS to serve as a frequency converter, for example, providing stable and accurate 60 Hz power from a 50 Hz power source, or from a power source having an unstable frequency such as an emergency generator. An additional advantage is that no switch is required to disconnect the input of the UPS from an a.c. source, since the inverter for inverting rectified power from the a.c. source will not feed power from other power supplies back through the rectifier.
This true UPS topology, using a dual conversion approach with the a.c. power being converted to d.c. power, and then back to a.c. power, is widely used in medium and large UPS systems, or those with output ratings above 20 kva or so, as well as small systems used in critical applications such as telecommunications.
The main disadvantage of this true UPS approach at lower power ratings has been poor efficiency. The reason for the poor efficiency of the true UPS in smaller sizes is the use of batteries, or other power sources, with relatively low d.c. voltages. Efficiency at low d.c. voltage is poor, not only because of ohmic conduction losses, but also because of losses in the semiconductor switching devices of the rectifier and inverter which have a relatively constant on-state voltage drop. A typical 1.0 volt drop across a conducting transistor, for example, is an insignificant 0.25% loss in a 400 volt apparatus, but represents a loss of nearly 4.2% in a 24 volt system.
The d.c. voltage utilized in a UPS is typically dictated by the cost of the batteries. For a given level of stored energy, a string of relatively few large cells is of significantly lower cost than a string of relatively many small cells. At the power levels typical of a small, single phase UPS in the 1 kva range, for example, batteries are most economical in the 24 to 36 volt range, but significantly higher efficiency would be achieved at a d.c. voltage in the 400 volt range.
U.S. Pat. No. 5,010,469 issued on Apr. 23, 1991 to Howard H. Bobry (the inventor herein) discusses the advantages and the disadvantages of a "true UPS". The patent discloses a UPS in which the load is normally supplied with power from an a.c. power line, with a battery supplying power upon failure of line power. The power line is connected through an isolation transformer to a rectifier which provides a relatively high voltage d.c. power source. This d.c. power source is connected to a single primary winding of a transformer through inverter circuitry having one input connected to the relatively high voltage d.c. power source and another input of the inverter circuitry connected to a low voltage d.c. power source, such as a battery, to effect a connection of the low voltage source through the inverter circuitry to a common portion of the transformer winding which is common to both d.c. sources. Operation of the inverter circuitry at the higher one of the two diverse input voltages is achieved through the use of taps on the single primary winding of the transformer and is such that the inverter circuitry operates to energize the primary winding from the relatively high voltage d.c. source as long as it maintains a higher voltage than that of the low voltage power source across the common part of the primary winding. Upon a failure of the high voltage source to maintain this higher voltage across the common portion of the primary winding, the low power source will supply the power to the primary winding until the high voltage power source again establishes a higher voltage across the common part of the primary winding
While the UPS of this prior patent achieves operation at two different voltages to provide an increase in efficiency, the isolation transformer, for the power supply input to the high voltage rectifier, adds to the size, weight, and cost of the system and reduces overall efficiency of the system. This isolation is needed because of the shared inverter circuitry and a common transformer primary winding. Thus an isolation transformer for the high voltage power source is required.
In addition, the change over voltage at which the UPS supplies power from a lower order voltage source is dictated by the voltage of the lower voltage power source so that a lower voltage source cannot be given preference over a higher voltage source for supplying power to the load. Moreover the UPS of the patent is not amenable to having any additional d.c. power source connected to the common inverter and common transformer winding.
Among the various objects of the present invention, which will be apparent from the description of preferred embodiments, is the provision of a true UPS topology which: (1) enables the use of one or more a.c. power supplies, including one or more high voltage a.c. power supplies, for establishing one or more high voltage d.c. power sources for the UPS; (2) enables the order of preference (priority) for d.c. power sources of the UPS to be in accordance with or different from the order of the voltage levels of the d.c. sources and in accordance with the magnitude of an effective voltage established for each d.c. power source; (3) enables the establishment of an order of preference for the d.c. power sources which have substantially the same voltage level; (4) enables an order of preference for the d.c. power sources to be established by effective voltages for the d.c. power sources which approximate a desired output voltage from the UPS; (5) enables the setting of the effective voltages to a voltage higher than a desired output voltage with the output voltages being regulated to the desired output voltage; (6) enables the easy addition of one or more power supplies to a UPS; (7) enables the maximizing of efficiency of the transistors and the inverters of the UPS as well as cost reduction in providing isolation for the power supplies from each other and the output of the UPS to thus maximize overall system efficiency while reducing costs.